Actually, i’m not sure that you can. You can ask for that for a soundfile using sfinfo~, but it seems info~ doesn’t offer the same feature. I suggest trying using cues via sflist~ and such anyway, as it’s not consuming so much more cpu than multiple buffers from my own experience : I came across the same problem earlier, and here’s a possibly buggy workaround i used first, before switching everything to the sflist~ system (also because i needed ram memory) :

— Pasted Max Patch, click to expand. —

Copy all of the following text. Then, in Max, select New From Clipboard.

the aim of this is to duplicate the ch1 output into where ch2 should go, if and only if there is strictly nothing coming out the ch2 output. If you don’t use a peakamp~ before checking if it’s == 0, it’s all buggy because zero crossing is obviously a common thing in time domain. There is probably a better way to detect if a signal is continuously equal to 0, in ways allowing to stay in signal time-domain (like with 2 thresh~maybe ?) but you get the idea.